1. Field of the Invention
The present invention relates to a pre-amplifier for optical receiving and an optical receiver using the same.
2. Description of the Related Art
In recent years, with the improved processing speed of communications systems and computers, optical signal communication between systems has been demanded. For such a communication system, small, low cost, adjustment-free optical receivers have been desired. Particularly, it has been desired to realize reduced cost and low power consumption in an optical communication system with a bit rate of less than 1 Gbps.
FIG. 29 is a block diagram showing the circuit configuration of an optical receiver. In FIG. 29, the optical receiving trans impedance-type pre-amplifier 1A, described later with FIG. 30, amplifies and outputs to a predetermined voltage signal a current signal converted from an optical signal by a light receiving element (photoelectric element such as a photo-diode) 2.
Numeral 3C represents a reference detector described later according to FIGS. 31 and 32. The reference detector 3C detects the center potential of an amplitude of an output signal from the pre-amplifier 1A and produces its potential as a standard reference potential for the amplifying operation of a limiter amplifier 4 to output to a limiter amplifier 4. The limiter amplifier 4 amplifies the output signal of the pre-amplifier based on the reference potential from the reference detector 3C.
The trans impedance-type pre-amplifier 1A, as shown in FIG. 30, is formed of four field effect transistors 10, 11A, 12, and 13A (hereinafter referred to FET), a feedback resistor (with a resistance value Rf) 14, and two diodes 20 and 21. The FET 11A acts as a load resistor and FET 13A acts as constant current source (load resistance).
In the pre-amplifier 1A, a current signal converted from an optical signal by the light receiving element 2 is inputted to the gate terminal 10a of the input FET 10 via an input terminal 18. A first power source (positive power source Vdd or GND 0 V) 16 is connected to the drain terminal (one signal terminal) 10b of the input FET 10 via the load FET 11A and a second power source (GND or negative power source Vss) 17 is connected to the source terminal (the other signal terminal) 10c via the diode 20.
The output FET 12 has a gate terminal 12a connected to the drain terminal 10b of the input FET 10, a drain terminal 12b connected to the first power source 16, and a source terminal 12c connected to the second power source 17 via the diode 21 and the load FET 13A. The amplified result amplified to a predetermined voltage value is outputted from the output terminal 19 connected to the source terminal 12c of the output FET 12.
A feedback resistor 14 supplies the output signal of the output FET 12 back to the gate terminal 10a of the input FET 10 and is arranged between the gate terminal 10a of the input FET 10 and the output terminal 19 (diode 21) of the output FET 12.
The load FET 11A has a gate terminal 11a and a source terminal 11c connected to the drain terminal 10b of the input FET 10 and to the gate terminal 12a of the output FET 12, and a drain terminal lib connected to the first power source 16. The load FET 13A has a gate terminal 13a and a source terminal 13c connected to the second power source 17 and a drain terminal 13b connected to a feedback resistor 14 and a diode 21.
In the trans impedance-type pre-amplifier 1A, a current signal Iin is supplied to the gate terminal 10a of the input FET 10 via the input terminal 18. The current signal is obtained by converting to an optical signal by the light receiving element 2 an optical signal representing a digital signal received by way of an optical fiber (not shown) from the transmission side. The potential of the drain terminal 10b of the input FET 10 is supplied to the gate terminal 12a of the output FET 12 and the potential of the source terminal 12c of the output FET 12 is outputted as an amplified result from the output terminal 19.
The reference detector 3C, as shown in FIG. 31, is constituted of a high level peak potential detector 3a for detecting a high level peak potential of an output signal of the pre-amplifier 1A, a low level peak potential circuit 3b for detecting a low level peak potential of an output signal of said pre-amplifier 1A, and an average value detector 3c for averaging the peak potentials from the above detectors 3a and 3b to output the averaged result as a predetermined standard reference potential to the limiter amplifier 4.
The high level peak potential detector 3a, as shown in FIG. 32(a), is formed of a diodes D1 and a capacitor C1. The low level peak potential detector 3b, as shown in FIG. 32(b), is formed of a diode D2 and a capacitor C2. The average value detector 3c, as shown in FIG. 32(c), is formed of two resistors R1 and R2 having the same resistance value to each other.
The pre-amplifier 1A shown in FIG. 30 has an output voltage to input current characteristic as shown in FIG. 33. For example, with the use of the power source voltage of 5 volts, the pre-amplifier 1A saturates with the output amplitude of more than 0.6 v. For that reason, an intensive light hitting the light receiving element 2 may vary the duty of the output waveform of the pre-amplifier 1A so that eye pattern (eye diagram) cannot be recognized because of its glared state.
The eye pattern is used to evaluate the degree of the inter-sign interference due to transmission distortion by using an aperture ratio (eye aperture: the unoverlapped portion when all possible pulse waveforms produced between two signal intervals are overlapped to display) of a waveform pattern which is overlapped on a cathode-ray tube a received and demodulated base band signal series by time-sweeping synchronously with bit. This procedure limits the dynamic range of the receiver.
Hence, as shown in FIG. 30, a diode is connected in parallel across the feedback resistor 14. This circuit configuration can limit the input current Iin to less than a fixed amplitude by making the diode conductive to form a by-pass circuit for an optical current when the input current Iin exceeds a fixed value.
However, in the above circuit, a sufficient eye aperture cannot be obtained because the output waveform duty of the pre-amplifier 1A varies in principle and the eye pattern is blinded. The above circuit configuration cannot be adopted to the system. The reason is that if the eye pattern does not have a sufficient eye aperture for a received signal corresponding to each optical signal, each channel cannot be recognized when each channel must be recognized with common clocks in the parallel transmission of plural channel optical signals.
As described above, there is a disadvantage in that the optical receiving pre-amplifier, as shown in FIG. 30, cannot establish a wide dynamic range and sufficient eye aperture because an insufficient output amplitude causes a waveform distortion due to an intensive light hitting the light receiving element 2.
In the optical receiver, as shown in FIG. 29, since the pre-amplifier 1A has a wide dynamic range, the reference detector 3C receives an input voltage with a large swing of amplitude. Most of the peak potential detectors 3a and 3b in the reference detector 3C are formed of a diode and a capacitor, as shown in FIGS. 32(a) and 32(b).
However, it is very difficult to obtain accurately the center potential (reference potential) over all levels in the configuration that the outputs of the peak potential detectors 3a and 3b are inputted to the average value detector 3c. Particularly, when the input level is small, the probability that a large error occurs is very high.
It is difficult that the optical receiving pre-amplifier establishes usually a sufficient eye aperture because of an error in a reference potential when an optical parallel transmitting receiver identifies plural received signals (channels) using common clocks, whereby plural received signals cannot be identified certainly. Since the capacitors C1 and C2 with larger capacitance value in the reference detector 3C can make the error small, an optical parallel receiver formed in an IC device requires a relatively large capacitor therein, thus resulting in an increased cost.
As described above, in the optical receiver shown in FIG. 29, since the reference detector 3C cannot detect accurately a reference to the dynamic range of an output of the pre-amplifier 1A, a sufficient eye aperture cannot be established at all times, whereby it is difficult to recognize each channel with common clocks at parallel receiving time.
There is a problem that a feedback in a circuit using a variable gain circuit may cause a possible oscillation. Moreover, in a system including plural optical receivers arranged in parallel, there is a disadvantage in that a reference detector 3C with a capacitor of large capacitance requires for each channel, thus resulting in a large-sized IC built-in circuit and an increased cost.